What exactly are you trying to do?
This
mask = g_channel > 100, b_channel > 800, r_channel < 800, b_channel < 0, r_channel < 0;
Is the same as this
mask = g_channel > 100, % only this part is assigned to mask
b_channel > 800, % this does nothing
r_channel < 800, % this does nothing
b_channel < 0, % this does nothing
r_channel < 0; % this does nothing
which, other than dumping a bunch of unused garbage to console, does the same as this:
mask = g_channel > 100;
It should stand to reason that not all colors described by this mask are green (e.g. [255 255 255]).
One might expect that this is a set of upper and lower thresholds for each color channel being applied, but the values and relations are nonsense for what I'm assuming is a uint8 image.
mask = g_channel > 100 & b_channel > 800 & r_channel < 800 & b_channel < 0 & r_channel < 0;
Why test that B and R are above/below 800, when they won't be above 255? How would B or R ever be less than zero?
Since you're asking how to count "green pixels", it's up to you to define the limits of what constitutes "green".
